Langmuir 1995,11, 1313-1317
1313
Organization of Au Colloids as Monolayer Films onto IT0 Glass Surfaces: Application of the Metal Colloid Films as Base Interfaces To Construct Redox-Active Monolayers Amihood Doron, Eugenii Katz, and Itamar Willner" Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel Received October 11, 1994. I n Final Form: January 3, 1995@ Au colloid films are organized on indium tin oxide (ITO) surfaces using (aminopropy1)siloxane or (mercaptopropy1)siloxane as base monolayer for the deposition of the metal colloid. Different Au colloids, ranging in particles of diameters 25,30, 35, and 120 nm, were deposited on the monolayer-modified IT0
surfaces. For the small particles, 25 nm, an almost continuous Au colloid film is formed with interparticle spacing of 10-25 nm. The surface coverage of the Au colloid on the (aminopropy1)siloxanemonolayer is higher than that for the (mercaptopropy1)siloxane-modifiedITO. The Au colloid films provide active surfaces for the self-assembly of redox-active thiolate monolayers. 8-(N-Methyl-4,4'-bipyridinyl)octanoicacid was covalently linked t o a cystamine monolayer assembled onto the Au colloids. For the 25-nm Au colloid the mol.cm-2, is ca. 12-fold higher than that of the surface coverage by the redox active unit, 6.8 x (aminopropy1)siloxanemonolayer-modified ITO, lacking the Au film. The surface coverages of the Au colloid films by the bipyridinium monolayers increase as the colloid particle sizes decrease.
Introduction Organization of miniaturized devices on monolayermodified metal surfaces, such as Au, is a subject of recent extensive research e f f ~ r t s . l - ~ Lithographic and photolithographic microstructuring of monolayers led to the selective microstructured association of biomaterials such as oligonucleotides,4 a n t i b ~ d i e s and , ~ cells6 onto metal surfaces. Organization of redox enzymes onto monolayermodified electrodes has established means to develop amperometric biosensors7 and reversible amperometric immun~sensors.~ Self-assembled monolayers on Au surfaces have also been applied to organize microstructured electronic circuits on nonconducting surfaces.8 Thin metal films,g and especially metal island films,l0 attracted considerable interest in view of their unique optical properties1' and potential applications. Secondharmonic generation,12 surface-enhanced Raman scatAbstract published inAdvance ACSAbstracts, March 15,1995. (1)(a) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A,; Sagiv, J. Nature 1988,332,426-429. (b) Haussling, L.; Ringsdorf, H.; Schmitt, F.-J.;Knoll, W. Langmuir 1991, 7, 1837-1840. (2) (a) Obeng, Y. S.; Bard, A. J. Langmuir 1991, 7, 195-201. (b) Hickman, J. J.; Ofer, D.; Zou, C.; Wrighton, M. S.; Laibinis, P. E.; Whitesides, G. M . J . A m . Chem. SOC.1991,113, 1128-1132. (c) Katz, E.; Schlereth, D. D.; Schmidt, H.-L. J . Electroanal. Chem. 1994,367, @
59-70. (3) (a) Tarlov, M. J . ; Burgess, D. R. F., Jr.; Gillen, G. J . Am. Chem. SOC.1993,115,5305-5306. (b) Huang, J.;Dahlgren, D. A,;Hemminger, J. C. Langmuir, 1994,10, 626-628. (4) Fodor, S. P. A.; Read, J. L.; Pirrung, M. C.; Stryer, L.; Tsai Lu, A.; Solas, D. Science, 1991, 251, 767-773. (5) Willner, I.; Blonder, R.; Dagan, A. J . Am. Chem. SOC.,in press. (6) Singhvi, R.; Kumar,A.; Lopez, G. P.;Stephanopoulos, G. N.; Wang, D. I. C.; Whitesides, G. M.; Ingber, D. E. Science 1944,264,696-698. (7) (a) Katz, E.; Riklin, A,; Willner, I. J . Electroanal. Chem. 1993, 354, 129-144. (bj Willner, I.; Riklin, A,; Shoham, B.; Rivenzon, D.; Katz, E.Adu. Mater. 1993,5,912-915. (c) Willner, I.; Riklin, A.;Anal. Chem. 1994, 66, 1535-1539. (d) Katz, E.; Schlereth, D. D.; Schmidt, H.-L.; J . Electroanal. Chem. 1994, 368, 165-171. (e) Kajiya, Y.; Okamoto, T.; Yoneyama, H. Chem. Lett. 1993, 2107-2110. ( 8 )Schoer, J . K.; Boss, C. B.; Crooks, R. M.; Corbitt, T. S.;HampdenSmith, M. J . Langmuir 1994, 10, 615-618. (9) (a) Winter, C. S.; Tredgold, R. H.; Hodge, P.; Khoshdel, E. IEE Proc.PartI:Solid-StateElectronDeuices 1984,131,125-128. (bjTarlov, J . J . Langmuir 1992, 8, 80-89. (c) Polymerpoulos, E. E.; Sagiv, J. J . Chem. Phys. 1978, 69, 1836-1847. (10) (a) Doremus, R. H. J . Appl. Phys. 1966, 37, 2775-2781. (b) Norrman, S.; Andersson, T.; Granqvist, C. G.; Hunderi, 0. Phys. Reu. B 1978, 18, 674-695. ( c ) Schimmel, T.; Bingler, H.-G.; Franzke, D.; Wokaun, A. Adu. Mater. 1994, 6 , 303-307. (11)Wokaun, A. Mol. Phys. 1985, 56, 1-33.
tering of absorbates,13 and the absorption and luminescence-of dye molecules14are controlled by the metal film thickness. Thin and transparent metal surfaces are also important as a base matrix for the organization of selfassembled monolayers since these allow the spectroscopic (absorption, luminescence) characterization of the monolayers. Chemical vapor deposition (CVD)and sputtering were used to generate transparent thin layers of gold on glass surface^.'^ To fabricate stable gold layers on glass surfaces, it is essential to use a primer for improved contact.16 The fabrication of transparent thin Au films on glass surfaces (5-10 nm thick) was recently described using titanium-primed g 1 a ~ s . lThese ~ transparent Au films were used as a base surface to immobilize and spectroscopically analyze self-assembled monolayers. An alternative method to generate thin Au films applied a thiol functionalized siloxane for the primary silanization of the glass surface.ls The resulting thiol-functionalized monolayer on the glass acted as primer layer for deposition of gold. Metal colloids, i.e., Au or Ag, bridge the atomic and bulk levels of metal properties. Unique optical and catalytic properties of metal colloids are controlled by the aggregation degree and size of the metal c l u ~ t e r s . 'The ~ high surface area of metal colloids suggests that their immobilization on surfaces could lead to the fabrication of thin metal-colloid films.20 Here we report on the (12) (a) Wokaun, A. Bergman, J. G.; Heritage, J. P.; Glass, A. M.; Lia0,P.F.; Olson,D. H.Phys.Reu.B 1981,24,849-856. (b)Aussenegg, F. R.; Leitner, A.; Pedaring, J. D. Appl. Phys. 1989, B49, 279-281. ( c ) Rundick, J.; Stern, E. A. Phys. Reu. B 1971, 4, 4274-4290. (13) Bergman, J.G.;Chemla,D.S.;Liao,P.F.;Glass,A.M.;Pinczuk, A,; Hart, R. M.; Olson, D. H. Opt. Lett. 1981, 6 , 33-35. (14) (a) Glass, A. M.; Liao, P. F.; Bergman, 3. G.; Olson, D. H. Opt. Lett. 1980,5, 368-370. (b) Meier, M.; Wokaun, A,; Liao, P. F. J . Opt. SOC.Am. E 1985,2, 931-949. (15) (a)Widrig, C. A,;Majda, M.Ana1. Chem. 1987,59,759-760. (b) Widrig, C. A.; Majda, M. Langmuir 1989,5, 689-695. ( c j Wehmeyer, K. R.; Deakin, M. R.; Wightman, R. M. Anal. Chem. 1986,57, 19131916. (d) Finklea, H. 0.;Avery, S.; Lynch, M.; Furtsch, T. Langmuir 1987,3,409-413. (e) Sabatini, E.; Rubinstein, I. J . Phys. Chem. 1987, 91, 6663-6669. (16) (a)Tarlov,M. J.; Newman, J . G.Langmuir 1992,8,1398-1405. (b) Wollman, E. W.; Frisbie, C. D.; Wrighton, M. S. Langmuir 1993,9, 1517-1520. (17) Dimillia, P. A,;Folkers, J. P.; Biebuyck, H. A,; Harter, R.; Lopez, G. P.; Whitesides, G. M. J . Am. Chem. SOC.1994, 116, 2225-2226. (18)Goss, C. A,; Charych, D. H.; Majda, M. Anal. Chem. 1991, 63, 85-88.
0743-7463/95/2411-1313$09.00/0 0 1995 American Chemical Society
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1314 Langmuir, Vol. 11,No. 4,1995
organization of Au colloid films on indium tin oxide (ITO) surfaces using an (aminopropy1)siloxane or a (mercaptopropy1)siloxaneas base monolayers for deposition of the metal colloid. The resulting Au colloid films exhibit high stability and allow further modification of the Au particles by functionalized self-assembled monolayers. The extent of surface modification of the Au film is controlled by the colloid size, and the transparency of the metal films allows the spectroscopic characterization of the Au particles and molecular adsorbates.
Experimental Section Light-scattering measurements were performed with a Malvern Autosizer 2C. AFM measurements were performed with AFM Model ARIS 3300, Burleigh Co., equipped with a 5-pm scanner. SEM experiments were executed with a JEOL 200 CX SEM attachment ASDID4D apparatus. Absorption spectra were recorded with an Uvikon-860 (Kontron) spectrophotometer. Electrochemical measurements were performed using a potentiostat (Versastat,EG&G)connected to a PC computer (Research Electrochemistry Software Model 2701250, EG&G). The electrochemical cell consisted of three electrodes where the I T 0 electrode, modified by the Au colloid and the respective redoxactive monolayer, was used as a working electrode, a carbon electrode separated by a frit from the working volume was used as counter electrode, and a saturated calomel electrode (SCE), connected to the working volume with a Luggin capillary, was used as reference electrode. The electrolyte solution consisted of a 0.01 M phosphate buffer solution, pH = 7.0, and 0.1 M Na2SO4 was used as supporting electrolyte. Benzoquinone (2-acetic acid) sulfide (2) was prepared by the reaction of 2-mercaptoacetic acid and benzoquinone in ethanol according to the literature.21 8-(N-Methyl-4,4'-bipyridinyl)octanoic acid (1)was prepared by the reaction of N-methyl(4pyridy1)pyridinium with 8-bromooctanoic acid in DMF (2 h of boiling). N-Methyl-N'-(3-aminopropane)-4,4'-bipyridinium (3) was prepared as described previously.22 All compounds gave satisfactory elementary analyses. All other materials were of commercial sources. The Au colloids were prepared according to the literature23by boiling a n HAUc14 aqueous solution with Na3 citrate. The resulting Au colloid sizes were controlled by the molar ratio of HAuClfla3 citrate. The molar ratios of HAuClfla3 citrate used for the preparation of the different colloids were as follows: colloid A, 0.20; colloid B, 0.33;colloid C, 0.43; colloid D, 0.66. The Au colloid films on I T 0 glass surfaces were prepared by primary modification of the surfaces with (aminopropy1)siloxane or (mercaptopropy1)siloxane. I T 0 glass plates 20 mm x 6 mm were washed with ethanol and dried. The plates were placed in a 1%dry toluene solution of (3-aminopropy1)trimethoxysilaneor (3-mercaptopropy1)trimethoxysilane.The solutions were heated overnight a t 110 "C, under Ar. The modified plates were rinsed with dry toluene and ethanol and dried. The plates were then immersed into the respective Au colloid for 2 h. The plates that included the Au colloid films were rinsed with water (the film was not affected by further rinsing with ethanol, DMF, or acetonitrile). Covalent attachment of 8-(N-methyl-4,4'-bipyridinyl)octanoic acid (1) to the Au colloid films was performed as previously (19)(a)Hulst, H. C. Light Scattering by Small Particles;Wiley: New York, 1957. (b) Henglein, A. J . Phys. Chem. 1979,83,2858-2862. (c) Henglein, A. Lilie, J. J . Am. Chem. SOC.1981,103,1059-1066. (d) Meisel, D.; Mulac, W. A.; Matheson, M. S. J . Phys. Chem. 1981,85, 179-187. (e) Kopple, K.;Meyerstein, D.; Meisel, D. J . Phys. Chem. 1980,84,870-875.(DThomas,J.M. PureAppl. Chem. 1988,60,15171528. (20)For previous studies directed toward the fabrication of ordered colloid monolayers, cf. (a) Giersig, M.; Mulvaney, P. Langmuir 1993, 9, 3408-3413. (b) Denkov, N. D.; Velev, 0. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K Langmuir 1992,8,3183. (21)Snell, J. M.; Weissberger, A. J . Am. Chem. Soc. 1939,61,450453. (22)Katz, E.; de Lacey, A. L.; Fierro, J. L. G.; Palacios, J. M.; Fernandez, V. M. J. Electroanal. Chem. 1993,358,247-259. (23)Turkevich, J.;Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951,11,55-75.
Scheme 1. Organization of a Monolayer-Gold-Colloid Electrode Modified with the Redox Active Viologen Groups
IT0
IT0
IT0
IT0
IT0
described for bulk Au electrodes.% The I T 0 plates modified by the Au colloid films were placed for 2 h in an aqueous solution of cystamine, 1 x M. The plates were then rinsed with water, ethanol, and DMF and introduced into a dry DMF solution M, that included 0-(benzotriazolof 1,5 x M, or 3,5x 1-y1)-NJVJV'JV'-bis(tetramethy1en)uronium hexafluorophosM, and ethyldiisopropylamine, EDPA, 3 phate, BBC, 1 x x 10-2 M. The plates were incubated in this solution for 1.5 h and then rinsed with DMF and water. The quinone- bipyridinium diad monolayer was attached to the Au colloid by a sequence of chemical transformations. The I T 0 plates modified by the Au colloid were placed for 30 min in a n ethanol solution of 2,2 x lom2M, and subsequently rinsed with ethanol and DMF. The resulting plates consisting of the quinone monolayer attached to the Au colloid films were then introduced to a dry DMF solution that included 3,5 x M, M, and EDPA, 3 x 10-2 M. The plates were BBC, 1 x incubated in this solution for 30 min and rinsed with DMF, ethanol, and water.
Results and Discussion Indium tin oxide glass surfaces were silanized by (aminopropy1)triethoxysilane and (mercaptopropy1)triethoxysilane, respectively, Scheme 1. The monolayer surface-modified I T 0 glass includes appropriate -NH2 and -SH functionalities capable of associating to Au interfaces. The surface densities of the amino and thiol monolayer on the I T 0 glass were estimated by reaction of the monolayer with 1. By following the charge associated with the reduction of the covalently-linked bipyridinium (l),the lower-limit for the surface coverage was estimated to be 0.5 x 10-lo and 0.02 x 10-lomol.cm-2 for the amino-modified monolayer and thiol-modified monolayer, respectively. The value for the aminopropyl monolayer coverage of I T 0 is in agreement with values of surface coverage of this monolayer determined by other methods.25 A series ofAu colloids(A-D) exhibiting different average diameters (in narrow size distributions) were prepared according to the literature.26 The size distributions of the different Au colloid samples were determined by light scattering, and their values are summarized in Table 1. Treatment of the monolayer-modified I T 0 glass surfaces with the aqueous Au colloids resulted in immediate deposition of the Au colloid onto the glass surface. The (24)Katz, E.;Itzhak, N.; Willner, I. Langmuir 1993,9,1392-1396. (25)(a)Untereker, D. F.; Lennox, J. C.; Wier, L. M.; Moses, P. R.; Murray, R. W. J . Electroanal. Chem. 1977,81,309-318.(b) Diaz, A. F.; Kanazawa, K. K. ZBM J . Res. Deu. 1979,23,316-329.(c) White, H. S.; Murray, R. W. Anal. Chem. 1979,51,236-239. (26)Frens, G. Nature (Phys. Sei.) 1973,241,21-22.
Au Colloid Films
Langmuir, Vol. 11, No. 4, 1995 1315
Table 1. Size Distribution of Au Colloid Films on Amino- and Mercapto-Functionalized Monolayers Associated with IT0 Surfaces and Surface Coverage of the Au Colloid Films by a Redox Active Monolayer redox active monolayer Au colloid coverage" size (nm) surface microcoverage moleculesLS scopy (particles-cm-2) mol.cm-2 d particle-' A NH2 15.6f 5 24 f 5b 4.8 x 1O'O 6.8 x 10-lo 8500 f 28 SH 15.6f 5 25 f 5" 4.5 f 1O*O 0.47 x 10-lo 580 f 9 B NH2 18.0 f 5 28 f 5' 4.0 x 1O'O 4.7 x 10-lo 5900 f 25 SH 18.0f 5 30 f 5" 3.8 x 1Olo 0.31 x 10-lo 380 f 8 C NH2 23.3 f 5 35 f 8b 3.3 x 10'O 2.2 x 10-lo 4000 f 20 SH 23.3 f 5 37 f 8b 2.6 x 1Olo 0.25 x 10-lo 600 f 7 D NH2 80.9 f 10 120 f 15' 2.3 x lo9 0.43 x 10-lo11300 f 35 SH 80.9 f 10 118 f 15b 1.8 x lo9 0.12 x 10-lo 4000 f 21 Determined by AFM. Determined by electron microscopy. For bipyridinium monolayer, Scheme 1. Area represents geometrical area of electrode surface. a
1
H loo nm
0.015
0.005
0 450
500
550
600
650
700
hlm Figure 1. Spectrum of a monolayer-gold-colloid electrode (A and D correspond to different colloid size).
colloid coating appears as a continuous red-violet film. The Au colloid film is stable and is not washed-off from the glass surface upon repeated rinsing with water. Figure 1shows the absorption spectrum of colloid A (diameter 24 nm) and colloid D (diameter 120 nm) deposited onto the IT0 glass that contains the (aminopropy1)siloxaneprimer monolayer. The absorption band of the larger colloid particles (A = 620 nm) is red-shifted by ca. 90 nm as compared to the absorption band of colloid A (A,,,= = 530 nm). The absorption bands of colloids B and C fall in between these two bands. These results are consistent with the recorded absorption spectra of Au colloids in solution, where the maximum absorbance band is redshifted as the colloid particles turn larger. Figure 2 shows the scanning electron micrographs of colloidsC and D associated with the (aminopropy1)siloxane monolayer. The light particles represent Au colloid units as evident from EDS measurements. Screening different areas of the surface revealed that colloids C and D generated discontinuous arrays in a nondensely packed configurationof the metal particles. The scanning electron micrographs of the smaller colloids (colloid A and B) revealed interesting observations. When different areas of the monolayer surface were screened, noncontrastic pictures that identify Au particles could not be detected. EDS analyses of these noncontrastic areas revealed, however, that Au particles are deposited in these areas. The atomic force micrograph of colloid A attached to the aminosiloxane monolayer associated with the I T 0 surface is shown in Figure 3. An almost continuous film of Au
H 200 m Figure 2. Scanning electron micrographs of monolayer-goldcolloid electrodes: (A, top) colloid D and (B, bottom) colloid C with different magnifications.
particles with a n average diameter of 25 nm is observed. Similar continuous films are formed by colloids A and B on the aminothiolsiloxane monolayer attached to ITO. A large number of particles appear tangent one to another, although other particles are separated by interparticle spacings in the range of 10-25 nm. This imperfect coverage is attributed to the roughness of the base I T 0 substrate. Defects in the chemical association of the monolayer to the substrate and/or structural perturbation in attachment of the Au colloid to this rough surface could lead to the incompletecoverageof the monolayer. It should be noted that in a control experiment that examined the I T 0 surface prior to Au association, substantially larger islands of rough surface domain were observed. We thus conclude that the small Au colloids generate continuous arrays of Au particles on the base monolayers where the larger Au colloids from discontinuous metal particle assemblies on the primary monolayers. Table 1 summarizes the size distribution of the Au colloids as determined by light-scattering measurements and electron microscopy or AFM of the deposited particles on the different base monolayers. We see that the surface coverage of the (aminopropy1)siloxanemonolayer by the Au colloids is ca. 27% higher than the respective coverage of the (mercaptopropy1)siloxanemonolayer. The conclusion that the aminopropyl monolayer acts as a n improved
Doron et al.
1316 Langmuir, Vol. 11,No.4,1995
interface for association of the Au colloids is general and is also supported by the electrochemical experiments (vide infra), The Au colloid monolayer provides an active interface for the immobilization of redox-active self-assembled monolayers. Scheme 1outlines the sequence of reactions to immobilize 1 onto the Au monolayer. A cystamine monolayerwas immobilized onto the Au colloids supported by the amino or thiol siloxane monolayer. Coupling of 1 to the cysteine monolayer resulted in the bipyridinium redox-active monolayer. The bipyridinium monolayer on the Au colloids is stable, and no noticeable change in the colloid monolayer could be detected along with the chemical modification steps or within the electrochemical experiments. Figure 4 shows the cyclic voltammograms of the bipyridinium units attached to colloids A-D films associated with the (aminopropy1)siloxanemonolayer. The magnitude of the electrochemicalresponse associated with the reversible reduction and oxidation of the bipyridinium units is controlled by the size of the Au colloid immobilized onto the siloxane monolayer. For the small sized Au colloid (colloidA, diameter 25 nm) a high amperometric response is observed, while for the large colloid particles (colloid D, diameter 120 nm), a low electrochemical response is detected. The cathodic (or anodic) peak currents of the bipyridinium units linked to the different colloids show a linear dependence with respect to the scan rates (i, = u), Figure 5. This confirms that the redox units are associated as monolayers onto the electrodes. The electron transfer rate constants for the bipyridinium units linked to the different Au colloid films were estimated by Laviron’s method.27 For all Au films a similar electron transfer rate constant, ca. 65 s-l, is observed.28 The charge associated with the one-electron reduction (or oxidation) wave of the bipyridinium units represents the molar coverage of the electrode surface by the redox(27) Laviron, E.J . Electroanal. Chem. 1979, 201, 19-28.
0.8 7
I
I
1
I
I
0.4 -
a
E
0-
Y
-0.4
-
-0.8
-
a I
-0.8
-0.6
-0.4
-0.2
0
E / V
Figure4. Cyclicvoltammograms of the monolayer gold colloid electrodes modified with covalently attached viologen groups: a-d correspond to different colloid sizes A-D; e corresponds to viologen groups attached directly to the aminosilane monolayer without any gold colloid. Potential scan rate was 1V-s-l.
active monolayer. With the surface coverage of the I T 0 electrode by the Au colloid film known, the surface density of the bipyridinium monolayer on the particles can be estimated. Table 1summarizes the surface coverage of the Au colloid films by the redox-active monolayer. For all of the Au colloid films the surface coverage of the redox monolayer on the colloids associated with the aminosiloxane monolayer is higher than on the respective colloids immobilized onto the mercaptosiloxane monolayer. For both classes of Au films, the surface coverage of the geometrical area of the electrodes by the redox monolayer increases as the Au colloid is smaller. These results correlated with the surface coverage of the I T 0 electrodes
Langmuir, Vol. 11,No. 4,1995 1317
Au Colloid Films
€ \
::
Y
't
4
/
3 1
-t
' t /0
2
4
6
8
1
v I v s-l
0
1
2
Figure 5. Dependence of peak current on the potential scan rate for the monolayer-gold-colloid (size A) electrode modified with viologen groups.
Figure 6. Cyclic voltammogram of the monolayer-gold-colloid (size A) electrode modified with the quinone-viologen diads. Potential scan rate was 600 MV-s-'.
by the Au colloids. Knowing the density of the Au colloids and their sizes, one can estimate the average number of monolayers associated with each particle, Table 1. It can be seen that the number of monolayer units associated with colloid A (ca. 24 nm) is higher than that bound to the colloids B and C. Thus, the monolayer associated with the small-size Au colloid film is more densely packed than the monolayers bound to the larger colloids. Figure 4 also shows the cyclic voltammogram of the redox-active units obtained upon direct coupling of 1 to the (aminopropy1)siloxane monolayer in comparison to the electrochemical response of the bipyridinium units immobilized onto the colloid A film through the cystamine monolayer. The density of the redox groups directly associated with the electrode surface corresponds to 0.5 x mol*cm-2,while upon deposition of the Au film the density of the redox units is increased to 6.8 x molcm-2. Thus, deposition of a n Au film allows the amplification of surface coverage by functionalized monolayers. Furthermore, the low surface coverage obtained upon direct coupling of the bipyridinium units to the (aminopropy1)siloxanemonolayer indicates that the high electrochemical response of the redox-active monolayers in the presence of deposited Au colloids originate from direct association of these monolayers to the Au particles, rather than to the coupling of the bipyridinium units to surface sites lacking Au colloids. Preliminary experiments indicate that the Au films provide an interface to construct more complex redox active monolayers, such as acceptor-diad monolayers. Scheme 2 outlines the sequence of transformations applied to assemble a quinone-bipyridinium monolayer on the Au colloid (colloid A) film supported onto the (aminopropy1)siloxane monolayer. The Au film was reacted with benzoquinone (2-acetic acid) sulfide (2). The resulting monolayerwas coupled to N-methyl-"-( 3-aminopropane)4,4'-bipyridinium (3),to generate the diad monolayer. Figure 6 shows the cyclic voltammogram of the resulting redox-active monolayer (pH of the electrolyte solutions is 7.0). It consists of two waves: one quasi-reversible wave at E" * -0.05 V with ill-defined anodic peak attributed to the reductiodoxidation of the quinone component, and a second reversible reduction wave at E" = -0.61 V, corresponding to the reductiodoxidation of the bipyridinium unit of the diad, eq 1. The charge transferred in
Scheme 2. Organization of a Monolayer-Gold-Colloid Electrode Modified with the Redox Active Diads, Quinone-Viologen
(28) The peak-to-peak separations are not influenced by the ohmic resistance associated with the electrolyte and the gold colloids attached to the monolayer. Much higher rate constants were evaluated for other systems with the same electrolyte and cell c ~ n f i g u r a t i o n .In ~ ~addition, the interfacial electron transfer rates to the bipyridinium redox components associated with the Au layers are similar to that obtained for bipyridinium aminosilane attached directly to ITO. Thus, the ohmic resistance resulting from the Au colloid does not affect AE,.
Y
IT0
IT0
?
m
the reduction of the quinone component is almost twice that associated with the reduction of the bipyridinium units (8.6 pC vs 3.3 pC, respectively). As the reduction of the quinone units involves two-electrons, we conclude that almost all of the primary quinone monolayer is transformed to the diad assembly. The surface coverage of the electrode surface by the diad monolayer is 2.8 x 10-1 molcm-2 (or ca. lo3 molecules-particle-'). Q-MAV2'
2e-
+ 2H+ QH,-mV2+
QH2-MAV
Conclusions We have described a novel method to generate stable Au colloid base monolayers attached to I T 0 glass surfaces. The surface densities of the Au particles generating the metal layer are controlled by the colloid size. With small particles (diameter 25 nm) continuous deposition of the colloid on the surface takes place. With larger colloids (diameter 120 nm) the Au particles are deposited as a discontinuous layer. For all the systems the (aminopropy1)siloxane base layer provided an improved interface for deposition of the Au layers, as compared to the (mercaptopropy1)siloxanemonolayer. The Au colloidlayer enables the organization of redox-active monolayers on the metal particles. Further experiments to organize diad assemblies on the Au colloid layers and to utilize the unique optical properties of these surfaces in the electrochemical and photochemical stimulation of second harmonic signals are underway in our laboratory. LA940791G
